In the present application, the terms “upstream” and “downstream” are defined relative to the normal flow direction of gas (from upstream to downstream) through the turbomachine. Furthermore, the term “turbomachine axis” is used to designate the axis of rotation of the rotor of the turbomachine. The axial direction corresponds to the direction of the turbomachine axis, and a radial direction is a direction perpendicular to said axis. Similarly, an axial plane is a plane containing the axis of the turbomachine and a radial plane is a plane perpendicular to said axis. Finally, unless specified to the contrary, adjectives such as “inner” and “outer” are used relative to a radial direction so that a (radially) inner portion or face of an element is closer to the axis of the turbomachine than is a (radially) outer portion or face of the same element.
The invention relates to a turbomachine of the type comprising:                an annular combustion chamber;        a centrifugal compressor;        an annular diffuser enabling the flow of gas leaving the compressor to be diffused, and directing this flow of gas towards the combustion chamber, the diffuser comprising: an upstream portion oriented radially and presenting diffusion passages connected to the outlet from the compressor; an intermediate portion that is elbow-shaped; and a downstream portion having a series of circularly spaced-apart flow-straightening vanes; and        an outer casing surrounding the outside of the combustion chamber and the downstream portion.        
A centrifugal compressor comprises a centrifugal impeller that serves to accelerate the gas passing therethrough, and thereby increase the kinetic energy of said gas.
The diffuser presents an annular space surrounding the compressor. The diffuser serves to slow down the speed of the gas leaving the compressor, and as a result increases its static pressure. Diffusers may be of the vane type or of the duct type.
Both of those types of diffuser comprise an annular upstream portion oriented radially and presenting a series of diffusion passages connected to the outlet of the compressor to pick up the accelerated gas leaving the compressor. The section of the diffuser passages increases progressively from the inside towards the outside so as to diffuse the flow of gas leaving the compressor. Diffusers of the vane type make use of a series of diffusion vanes spaced apart circularly and forming diffusion passages between one another. In diffusers of the duct type, the diffusion passages are constituted by pipe or duct elements.
Downstream from said upstream portion, diffusers have an annular intermediate portion that is elbow-shaped for curving the flow path of the diffuser and for causing the gas to flow towards the combustion chamber.
Downstream from the intermediate portion, a diffuser has an annular downstream portion with a series of circularly spaced-apart flow-straightening vanes for straightening the flow of gas and thus reducing the gyratory component of the gas flow leaving the diffusion passages, prior to the flow entering the combustion chamber.
FIG. 1 shows a known example of a turbomachine of the above-described type that comprises, in the gas flow direction: a centrifugal compressor 110; an annular diffuser 120 having diffusion vanes 123; and an annular combustion chamber 140. An outer casing 132 surrounds the outside of the combustion chamber 140 and of the diffuser 120.
The diffuser 120 comprises: an upstream portion 121 that is radially oriented, presenting diffusion passages 122; an elbow-shaped intermediate portion 124; and a downstream portion 125 comprising a series of circularly spaced-apart flow-straightening vanes 126.
The flow stream is defined as being the space in which the gas flows.
The flow stream in the upstream and intermediate portions 121 and 124 is delimited by a first end plate 127 and a second end plate 128. The flow stream in the downstream portion 126 is delimited internally by the first end plate 127 and externally by an outer shroud 129.
The downstream portion 125 of the diffuser is oriented parallel to the axis A of the turbomachine. In other words, in a section plane containing the axis A of the turbomachine, the mean axis M of the flow stream leaving the downstream portion 125 of the diffuser is parallel to the axis A of the turbomachine. In this configuration, the main gas stream leaving the diffuser passes externally around the combustion chamber 140.
In an alternative that is not shown, in order to cause the main gas stream leaving the diffuser 120 to be directed towards the combustion chamber 140, so that the combustion chamber is thus better fed with gas, the downstream portion 125 of the diffuser is inclined relative to the axis A of the turbomachine so as to point towards the combustion chamber 140. In other words, in a section plane containing the axis A of the turbomachine, the mean axis M of the flow stream leaving the downstream portion 125 of the diffuser forms a (non-zero) acute angle with the axis A.
The drawbacks of known diffusers lie in the fact that the parts making them up are difficult to assemble together. In particular, when the downstream portion 125 is inclined relative to the axis A, it is particularly difficult to fasten the outer shroud 129 by brazing, or by crimping onto the downstream portion 125, since the outer surface of the downstream portion 125 is conical and the outer shroud 129 tends to slide towards the free end of the downstream portion 126.